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Direct Conversion of Methane to Methanol by Metal-Exchanged ZSM‑5 Zeolite (Metal = Fe, Co, Ni, Cu) M. Haris Mahyuddin,†,‡ Aleksandar Staykov,§ Yoshihito Shiota,† and Kazunari Yoshizawa*,† †

Institute for Materials Chemistry and Engineering and IRCCS, Kyushu University, Fukuoka 819-0395, Japan Department of Physics-Energy Engineering, Surya University, Tangerang 15810, Indonesia § International Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka 819-0395, Japan ‡

S Supporting Information *

ABSTRACT: Metal-exchanged zeolites are known to exhibit catalytic activity in the direct conversion of methane to methanol. The influence of different metals on this reaction has been theoretically investigated by using density functional theory (DFT) calculations on a periodic system of MO+-ZSM-5 zeolite (M = Fe, Co, Ni, Cu). The results indicate a high dependence of the reaction on the metals, where the reactivity toward C−H bond dissociation is predicted to increase in the order CoO+ZSM-5 < NiO+-ZSM-5 < FeO+-ZSM-5 < CuO+-ZSM-5 and the selectivity of methanol is predicted to increase in the order FeO+-ZSM-5 < CoO+-ZSM-5 < NiO+-ZSM-5 < CuO+-ZSM-5. The role of ZSM-5 zeolite in the catalytic activity is also investigated by comparing our calculation results with those reported for the reaction by bare MO+ species in the gas phase. We found that the nanopores of ZSM-5 zeolite exert a confinement effect which destabilizes the adsorption of methane and lowers the activation energy for the C−H bond dissociation. In addition to the conversion of methane, we investigated the direct conversion of ethane to ethanol by FeO+-ZSM-5 and found that this reaction proceeds with a lower C−H bond activation energy and a higher product selectivity in comparison to the conversion of methane to methanol by the same catalyst. KEYWORDS: methane hydroxylation, methanol, transition metals, ZSM-5 zeolite, DFT calculation

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INTRODUCTION The existing commercial conversion of methane for producing methanol involves a two-step process through the formation of synthesis gas (syngas), a mixture of CO and H2, followed by a catalytic process.1 Although this method has comparatively high yield methanol production,2,3 it is costly and sensitive to sulfur poisoning.1 In contrast, direct conversion has distinct economic and environmental advantages because it circumvents the expensive and environmentally harmful syngas step.4 Metalexchanged zeolites are known to exhibit a high catalytic activity in the direct conversion of methane to methanol. The pioneering work was reported by Panov and co-workers, who discovered a remarkably high reactive surface α-oxygen on a N2Oactivated Fe-ZSM-5 zeolite5 that is able to directly convert benzene to phenol6,7 and methane to methanol,8,9 in which the latter was achieved at room temperature with a selectivity of nearly 80%.8 They suggested that iron, referred to as the metal active site, is responsible for such catalytic properties of Fe-ZSM-5 zeolite.10 Following the success of Panov and co-workers, many researchers have been interested in exchanging different transition metals into ZSM-5 zeolite to achieve higher selectivity of methanol from activation of methane at low temperature. The significant © XXXX American Chemical Society

work was reported by Schoonheydt and co-workers, who experimentally investigated the conversion of methane over O2-activated Cu-ZSM-5 and achieved a methanol selectivity of >98% at low temperature (100 °C).11,12 The O2-activated Co-ZSM-5 was also reported to convert methane to methanol at 150 °C, but it has been less studied due to low yield and low selectivity of methanol.13,14 Therefore, the recent experimental works in the methane to methanol conversion involving metalexchanged ZSM-5 have been focused mainly on Fe-ZSM-515−17 and Cu-ZSM-5.18−20 From a theoretical point of view, density functional theory (DFT) calculations have suggested some possible structures of the iron21−26 and copper27−33 active sites in ZSM-5 zeolite: namely, the monoiron cation [FeO]+,21 the monoiron dication [FeO]2+,23 the bent mono(μ-oxo)diiron [Fe2(μ-O)]2+ and -dicopper [Cu2(μ-O)]2+,25,28 the bis(μ-oxo)diiron [Fe2(μO)2]2+,25 and the tricopper [Cu3(μ-O)3]2+.33 The bis(μoxo)dicopper [Cu2(μ-O)2]2+ was also proposed from observations of in situ XASF and UV−vis−near-IR spectroscopy,34 Received: June 17, 2016 Revised: October 29, 2016 Published: November 4, 2016 8321

DOI: 10.1021/acscatal.6b01721 ACS Catal. 2016, 6, 8321−8331

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Figure 1. (a−c) Periodic structures of an MO+ species sitting on the 10-membered ring of ZSM-5 zeolite (AlSi95O192) with the Al atom located at the (a) T1, (b) T2, and (c) T12 sites. (d) Optimized structures of FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 in the corresponding high-spin state, considered for the main discussion. Some atoms of the zeolite’s unit cell in (b) are omitted for clarity. SM stands for spin multiplicity. Bond lengths are given in Å.

of the methane to methanol conversion as well as to understand how the ZSM-5 nanopores influence the catalytic activity of MO+ toward methane. In addition, we also investigate the direct conversion of ethane to ethanol by FeO+-ZSM-5. Ethane is the second major component of natural gas after methane and has a lower C−H bond strength than does methane (101 versus 105 kcal/mol).47 Moreover, Hutchings and co-workers demonstrated that the selectivity of oxygenates produced from ethane oxidation by H2O2-activated Fe-ZSM-5 (97%)48 is higher than that produced from methane oxidation by the same catalyst (83%).17

where it was recently demonstrated from DFT calculations that such dicopper species, in the presence of a water molecule, forms a monocopper CuO species that can activate a C−H bond of methane with a lowered barrier.31 Despite the importance of the structure of the metal active site, the roles of metals and zeolite nanopores in the mechanism and energetics of the methane to methanol conversion by metal-exchanged ZSM-5 zeolite have been unexplored. Metal oxide cations that exclude the zeolite framework (bare MO+), on the other hand, have been long studied and known to have a particular role in the direct conversions of methane to methanol35−42 and benzene to phenol43−46 in the gas phase. Schwarz and co-workers demonstrated that the efficiency of the gas-phase reaction of methane with MO+ cations and the methanol branching ratio depend highly on the metals.35−38 For example, MnO+ and PtO+ react with methane very efficiently, but their branching ratios to methanol are only 1% and 25%, respectively. In contrast, FeO+, CoO+, and NiO+ do not react with methane as efficiently as the first two cations, but they have high methanol branching ratios of 41%, 100%, and 100%, respectively. Yoshizawa and co-workers theoretically confirmed that the activation energies for the C−H bond dissociation and the OH−CH3 recombination involved in the reaction are the key factors that determine the reaction efficiency and methanol branching ratio, respectively.39−42 In the present study, we carry out a comprehensive theoretical analysis of the energetics and mechanisms of the methane to methanol conversion by a variety of MO+-ZSM-5 complexes (M = Fe, Co, Ni, Cu). Such a simple mononuclear metal model was predicted as one of the possible structures of Fe-ZSM-521 and Cu-ZSM-531 and has been viewed as an effective mediator from the works by the Schwarz group.35−38 Although this model might be different from the actual structure and thus comparisons with experiments are difficult, this work provides a useful guidance for the development of methane hydroxylation by metal-exchanged ZSM-5 zeolites. Therefore, our main purposes in this work are to elucidate the influences of different metals on the mechanism and energetics

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METHODOLOGY Computational Methods. All DFT calculations were performed under the Kohn−Sham formulation49,50 as implemented in the Vienna ab initio simulation package (VASP).51,52 The projector augmented wave (PAW) method was employed to describe the interaction between ion cores and electrons.53,54 The electron exchange−correlation was treated by the generalized gradient approximation (GGA) based on the Perdew, Burke, and Ernzerhof (PBE) functional.55 Plane wave basis sets with a cutoff energy of 550 eV were used for all calculations. Brillouin zone sampling was restricted to the Γ point. The conjugate gradient method was used for geometry optimizations. The climbing-image nudged elastic band (CI-NEB) method with the quasi-Newton algorithm was used to determine a minimum energy path and to locate a first-order saddle point that corresponds to a transition state.56,57 A good estimate of the initial reaction path was generated by the imagedependent pair potentials (IDPP) method.58 During the calculations, all atoms of the zeolite and the active site as well as the molecules were allowed to fully relax, and the structures were considered as reaching convergence when the maximum force on unconstrained atoms was less than 0.03 eV/Å. To anticipate the involvement of two different spin states along the reaction, termed two-state reactivity by Shaik, Schwarz, and their co-workers,59 we considered spin-polarized calculations with two fixed spin multiplicities corresponding to 8322


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ACS Catalysis the ground and the first excited states of the initial MO+-ZSM-5 complexes. To account for van der Waals (vdW, dispersion) correction, single-point energy calculations using Grimme’s semiempirical D2 method were carried out.60 The structures optimized by the Kohn−Sham DFT calculations were used in these calculations. All parameters for each element were taken from Grimme’s original paper.60 Atomic charges and spin densities were calculated by using the grid-based Bader analysis algorithm developed by Henkelman and co-workers.61,62 The optimized structures and geometrical parameters were respectively visualized and measured by using VESTA.63 Model for MO+-ZSM-5. The framework type MFI of ZSM-5 zeolite, which is built by using units of 12 SiO4 tetrahedra (T) positioned at 12 distinct T sites, is characterized by a threedimensional pore system with straight channels in the [010] direction intersected by the zigzag channels in the [100] direction.64,65 A single unit cell of ZSM-5 zeolite (Si96O192) retrieved from the zeolite database66 was used in the present study. We optimized the structure and found the following cell parameters: a = 20.406 Å, b = 20.142 Å, and c = 13.522 Å, which are in good agreement with the experimental values of a = 20.07 Å, b = 19.92 Å, and c = 13.42 Å.64 One of the Si atoms was substituted by an Al atom, which thus gave a Si/Al ratio of 95. The negative charge resulted by this atom replacement was compensated by an MO+ cation, where M includes Fe(III), Co(III), Ni(III), and Cu(III). Schröder and Nachtigallová separately reported that there are no significant differences in energy between the ZSM-5 frameworks with Al atom at different T sites,67,68 although the recent works suggested that the energetics of adsorbates on various T sites differ substantially.69,70 Therefore, in this study we considered three different T sites located at the channel intersection, namely the T1, T2, and T12 sites. As shown in Figure 1a−c, an MO+ species sits on the wall of the 10-membered ring of ZSM-5 zeolite (AlSi95O192) with the Al atom at the T1, T2, and T12 sites, respectively. Table S1 in the Supporting Information shows that the initial FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 complexes in the corresponding highspin state with the Al atom at the T12 site are more stable by 1.3 (4.7), 0.9 (3.8), 1.1 (5.2), and 1.8 (3.7) kcal/mol than those at the T2 (and T1) sites, respectively. However, for our main discussion, we selected the T1 site as the location for the Si → Al substitution because this site gives the most stable adsorption of methane, as will be discussed in more detail in the Results and Discussion. As shown in Figure 1d, the lowest-energy state for FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 complexes is the sextet (spin multiplicity, SM = 6), quintet (SM = 5), quartet (SM = 4), and triplet (SM = 3) states, respectively. These high-spin states are 1.0, 7.9, 4.6, and 7.0 kcal/mol lower in energy than the corresponding first excited state, i.e. quartet, triplet, doublet, and singlet low-spin states, respectively. The results for FeO+-ZSM-5 are in good agreement with the work of Li et al., showing that a periodic system of FeO+-ZSM-5 in the sextet state is by 5.5 kcal/mol more stable than that in the quartet state.25Figure 1d also shows the optimized structures of FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 in the corresponding high-spin state, while those in the corresponding low-spin state are provided in the Supporting Information. The geometries of the isolated methane, ethane, methanol, and ethanol molecules were optimized by using a large cell to minimize the effect of interactions between the periodically repeated species. The C−H bond length and H−C−H angle

for methane (and methanol) are calculated to be 1.096 Å (1.100 Å) and 109.5° (108.6°), respectively. The calculated C−C and C−H bond lengths for ethane (and ethanol) are 1.542 Å (1.516 Å) and 1.098 Å (1.098 Å), respectively. In addition, the C−O and O−H bond lengths for methanol (and ethanol) are calculated to be 1.431 Å (1.439 Å) and 0.971 Å (0.972 Å), respectively. Reaction Pathway. As shown in Scheme 1, we consider a reaction pathway that proceeds in a two-step manner via two Scheme 1. Possible Catalytic Cycle for the Methane to Methanol Conversion by MO+-ZSM-521,22

transition states (TSs): MO+-ZSM-5 + CH4 (dissociation limit) → [MO(CH4)]+-ZSM-5 (reactant complex) → TS1 → [HOM−CH3]+-ZSM-5 (hydroxo intermediate) → TS2 → [M(CH3OH)]+-ZSM-5 (product complex) → M+-ZSM-5 + CH3OH (final complex).21,22 In this reaction, the O(oxo) species is inserted into a C−H bond of methane to release an M+ species. Therefore, to regenerate the O(oxo) species as well as to reoxidize the M+ species back to M3+, a decomposition of N2O, which is not the scope of the present study, is required as an additional process for completing the catalytic cycle of the direct conversion of methane to methanol by MO+-ZSM-5.

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RESULTS AND DISCUSSION Mechanism of Methane Activation by MO+-ZSM-5. In the first half of the reaction, as shown in Scheme 1, a methane molecule is reacted with MO+-ZSM-5 to form an adsorbed methane called reactant complex [MO(CH4)]+-ZSM-5. Afterward, a C−H bond of the methane molecule dissociates via TS1 and then the reaction intermediate [HOM-CH3]+-ZSM-5 is formed. Parts a−d of Figure 2 show potential energy diagrams for the reaction by FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 complexes, respectively, along the reaction pathway in the high-spin and low-spin states. As shown in these diagrams, the reactant complexes of [FeO(CH4)]+-, [CoO(CH4)]+-, [NiO(CH4)]+-, and [CuO(CH4)]+-ZSM-5 are preferably formed in the highspin state with the methane molecule adsorbed by a weak binding energies EB of −4.5, −3.5, −1.8, and −1.1 kcal/mol, respectively, where the first value is consistent with the reported value of −3.9 kcal/mol, calculated in a small cluster model.21 If the dispersion correction is taken into account, these values are reduced to −9.6, −8.3, −7.4, and −6.1 kcal/mol, respectively 8323


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Figure 2. Potential energy diagrams for the methane to methanol conversion by (a) FeO+-ZSM-5, (b) CoO+-ZSM-5, (c) NiO+-ZSM-5, and (d) CuO+-ZSM-5 along the reaction pathway in the high-spin state (blue lines) and the low-spin state (red lines). Values in parentheses are relative energies which include dispersion correction. Green lines show the relative energies for the formation of methyl radical M(OH)+-ZSM-5 + CH3•. All energies are given in kcal/mol.

Table 1. Binding Energies of Methane (EB), Relative Activation Energies for C−H Bond Dissociation via TS1 (ΔETS1), Relative Activation Energies for HO−CH3 Recombination via TS2 (ΔETS2), and Relative Energies for Methanol Desorption (ΔEMeOH) by MO+-ZSM-5 and Bare MO+ a previous workf

present work +

EBb ΔETS1c ΔETS2d ΔEMeOHe

+

+

+

+

FeO -ZSM-5

CoO -ZSM-5

NiO -ZSM-5

CuO -ZSM-5

FeO -ZSM-5

bare FeO

−4.5 12.9 37.3 30.3

−3.5 17.3 33.5 35.3

−1.8 15.8 19.0 32.8

−1.1 6.4 10.2 36.9

−3.9 16.0 41.6 26.4

−22.8 22.1 28.6 39.4

+

bare CoO+

bare NiO+

bare CuO+

−25.4 30.9 24.6 44.8

−28.9 25.4 21.3 56.8

−45.8 32.8 3.7 52.2

a

All energies are presented in kcal/mol. bEB is measured from the dissociation limit in the ground state to the reactant complex in the ground state. ΔETS1 is measured from the reactant complex in the ground state to TS1 in the ground state. dΔETS2 is measured from the hydroxo intermediate in the ground state to TS2 in the ground state. eΔEMeOH is measured from the product complex in the ground state to the final complex in the ground state. fReported values for the reaction by FeO+-ZSM-5 in a small cluster model21 and by bare MO+ in the gas phase40,41 were calculated by using 6-31G** and 6-311G** basis sets, respectively, at the B3LYP level. c

O(oxo) atoms, which are calculated to be ∠O−M−C = 111, 108, 97, and 85°, respectively. Interestingly, these values seem to have a good relevance to the binding energies, where the smaller the ∠O−M−C angle, the weaker the binding energy. This angle is related to the confinement effect exerted by the ZSM-5 zeolite, which influences the adsorption behavior and adsorption strength of methane (the details are discussed in Confinement Effect of ZSM-5 Zeolite on the Reactivity). Thus, such an effect explains the discrepancy in the trend of binding energies, discussed above. After the adsorbed methane was significantly activated by MO+-ZSM-5, a C−H bond of the methane weakens. Such a

(Figure 2, values in parentheses), but the energy trend remains unchanged because the amounts of the energy reduction are quite similar (about 5 kcal/mol). Interestingly, this energy trend is the opposite of that in the gas-phase reaction by bare MO+ cations (EB = −22.8, −25.4, −28.9, and −45.8 kcal/mol, respectively, for bare FeO+, CoO+, NiO+, and CuO+;41 see Table 1). As expected from the weak binding energies, the methane molecule in the reactant complexes of [FeO(CH4)]+-, [CoO(CH4)]+-, [NiO(CH4)]+-, and [CuO(CH4)]+-ZSM-5 is adsorbed with calculated M···C distances of 2.67, 2.63, 2.95, and 3.36 Å, respectively. Moreover, as shown in Figure 3, the C atom of these reactant complexes forms an angles with the M and 8324


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Figure 3. Optimized structures of reactant complex, TS1, and hydroxo intermediate for the methane to methanol conversion by (a) FeO+-ZSM-5, (b) CoO+-ZSM-5, (c) NiO+-ZSM-5, and (d) CuO+-ZSM-5 in the corresponding ground state. Some atoms of the zeolite’s unit cell are omitted for clarity. Bond lengths are given in Å. Color code: blue (Si), green (Al), red (O), gray (C), white (H), orange (Fe), purple (Co), yellow (Ni), cyan (Cu). A table showing the complete geometrical parameters in the high-spin and low-spin states is available in Supporting Information.

highly possible to occur in the activation of the C−H bond of methane.72 The SOC may affect the kinetics73 and energetics71,74 of the reaction, but Xiao et al. demonstrated that SOC does not affect the relative stability of gold clusters because it increases all the calculated binding energies by the same magnitude.75 As summarized in Table 1, the relative activation energies for C−H bond dissociation via TS1 are ΔETS1 = 12.9, 17.3, 15.8, and 6.4 kcal/mol, respectively for TS1s of FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5, where the value for FeO+-ZSM-5 is reasonably lower by 3.1 kcal/mol than that calculated in a small cluster model.21 Figure 2 shows that the ΔETS1 values with dispersion correction are 12.3, 17.1, 15.7, and 7.2 kcal/mol, respectively, which are not significantly different from those without dispersion correction. This suggests that ΔETS1 is not significantly influenced by dispersion forces, unlike the methane binding energy discussed above. On the basis of these ΔETS1 values, the reactivity of MO+-ZSM-5 toward C−H bond dissociation of methane is predicted to increase in the order CoO+-ZSM-5 < NiO+-ZSM-5 < FeO+-ZSM-5 < CuO+-ZSM-5. This reactivity order agrees with that in the gas-phase reaction by bare MO+,41 describing that such high reactivities of FeO+ and CuO+ complexes are due to their low-lying d-block orbitals, which significantly interact with the HOMO (highest occupied molecular orbital) of the CH4 fragment. Furthermore, the

weakened C−H bond thus facilitates the H atom to dissociate from the C atom and then to approach the O(oxo) atom via TS1, while the resultant CH3 moiety approaches the metal center. In Figure 2, crossings between the two potential energy surfaces having different spin multiplicities, referred to as the spin inversion, are observed before the TS1s of FeO+-, CoO+-, and NiO+-ZSM-5 and after the TS1 of CuO+-ZSM-5. This suggests that the C−H bond dissociation by FeO+-, CoO+-, and NiO+-ZSM-5 occurs along with an inversion of the corresponding ground high-spin state to the low-spin state, whereas that by CuO+-ZSM-5 occurs in the high-spin state without the spin inversion. As shown in Figure 3, the separating C···H distances for TS1s of FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 in the corresponding ground state are calculated to be 1.34, 1.32, 1.21, and 1.32 Å, respectively, while the approaching O···H distances are calculated to be 1.35, 1.37, 1.52, and 1.22 Å, respectively. The Fe···C, Co···C, Ni···C, and Cu···C distances of TS1s are calculated to be 2.12, 2.10, 2.16, and 3.14 Å, respectively. Such spin inversion was systematically discussed by Poli and Harvey,71 showing that the possibility and the mechanism of the spin inversion are highly dependent on the magnitude of the spin−orbit coupling (SOC). Previously, Shiota and Yoshizawa analyzed the magnitude of SOC in the direct methane to methanol conversion by bare FeO+, and the results showed that the spin inversion from the sextet state to the quartet state is 8325


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ACS Catalysis MOOP (molecular orbital overlap population) analysis for CuO+ shows a significant C−H bonding in the unoccupied orbitals, which makes it more effectively activate methane.41 Recently, Yumura et al. reported that a CuO species can be formed as a reaction intermediate (HO−Cu−OH−CuO) in the conversion of methane to methanol by Cu2O2-ZSM-5 if a water molecule is present in the reaction.31 They found that this intermediate species activates a C−H bond of methane with a lower barrier (ΔETS1 = 7.9 kcal/mol) than the anhydrous Cu−O−Cu species reported by Woertink et al. (ΔETS1 = 18.5 kcal/mol).28 Our result for CuO+-ZSM-5 (ΔETS1 = 6.4 kcal/mol) is consistent with this lowered barrier, suggesting that the presently considered mononuclear model of MO+-ZSM-5 can possibly form in the conversion of methane to methanol by Cu-ZSM-5 that involves water. The abstracted H atom is then covalently bound to the O(oxo) atom, while the CH3 moiety migrates closer to the metal center to form a covalent M−CH3 bond, resulting in a four-centered hydroxo intermediate [HOM-CH3]+-ZSM-5. Figure 3 shows that the formed O−H bond lengths for the [HOFe-CH 3 ] + -, [HOCo-CH 3 ] + -, [HONi-CH 3 ] + - and [HOCu−CH3]+-ZSM-5 intermediates in the low-spin ground state are calculated to be 0.97, 0.98, 0.98, and 0.97 Å, respectively, and the formed Fe−CH3, Co−CH3, Ni−CH3, and Cu− CH3 bond lengths are calculated to be 1.96, 1.91, 1.92, and 1.93 Å, respectively. Those calculated O−H bond lengths are in good agreement with the experimentally observed covalent O−H bond length of 0.97 Å.76 As shown in Table 2, significant decreases in O atom spin density from 0.88, 1.04, 1.16, and 1.26, respectively, for reactant complexes of [FeO(CH4)]+-, [CoO(CH4)]+-, [NiO(CH4)]+-, and [CuO(CH4)]+-ZSM-5 in the high-spin state to 0.14, 0.14, 0.15, and 0.0, respectively, for the corresponding hydroxo intermediates in the low-spin ground state are evident from such formation of covalent O−H bonds. Owing to such covalent O−H and M−CH3 bonds, the hydroxo intermediates in the low-spin ground state are low in energy (Figure 2). In particular, the [HOCu-CH3]+-ZSM-5 intermediate has the lowest energy, although its O−H and M−CH3 bond lengths are not significantly different from those of the other hydroxo intermediates. Figure S5 in the Supporting Information shows that the [HOCu-CH3]+-ZSM-5 intermediate in the singlet ground state, in comparison to the other hydroxo intermediates in the corresponding ground state, has a distinct structure, where the OH and CH3 ligands are very close to the mouth of the 10-membered ring of the zeolite. Moreover, as shown in Figure 3, the formed ∠O−M−C angles of 97.4, 97.3, 93.2, and 89.8°, respectively, for [HOFe-CH3]+-, [HOCo-CH3]+-, [HONi-CH3]+- and [HOCu-CH3]+-ZSM-5 intermediates in the low-spin ground state seem to have a good relevance to the stability of hydroxo intermediates (−23.0, −23.7, −30.0, and −37.9 kcal/mol, respectively), suggesting that the hydroxo intermediate is stabilized with the decreasing ∠O−M−C angle. The energetics of the dissociation limit and reactant complex with the Al atom located at T1, T2, and T12 sites of the ZSM-5 framework are shown in Table 3. We have discussed in the previous section that the initial MO+-ZSM-5 complexes with the Al atom at the T12 site are more stable than those at T1 and T2 sites. However, Table 3 shows that [FeO(CH4)]+-, [CoO(CH4)]+-, and [NiO(CH4)]+-ZSM-5 reactant complexes with the Al atom at the T1 site in the high-spin ground state are more stable by 1.2 (1.3), 0.5 (0.3), and 0.7 (0.2) kcal/mol, respectively, than those with Al atom at the T2 (and T12) sites

Table 2. Atomic Charges and Spin Densities for C, M, and O(oxo) Atoms of Reactant Complex TS1, Hydroxo Intermediate TS2, and Product Complex in the Corresponding Ground State atomic charge C reactant complex TS1 intermediate TS2 product complex

+0.30 +0.49 +0.37 +0.18 −0.39

reactant complex TS1 intermediate TS2 product complex

+0.26 +0.46 +0.28 +0.16 −0.31


+0.22 +0.43 +0.24 +0.11 −0.30


+0.22 +0.27 +0.24 +0.13 −0.35

M

atomic spin density O

FeO+-ZSM-5 +1.36 −0.73 +1.23 −0.77 +1.22 −1.09 +0.96 −1.03 +0.76 −1.16 CoO+-ZSM-5 +1.23 −0.62 +1.09 −0.71 +1.09 −1.01 +0.86 −0.97 +0.70 −1.13 NiO+-ZSM-5 +1.11 −0.51 +1.02 −0.63 +0.98 −1.01 +0.82 −0.96 +0.67 −1.15 CuO+-ZSM-5 +1.02 −0.42 +0.95 −0.67 +0.95 −0.98 +0.81 −0.91 +0.66 −1.08

C

M

O

0.04 −0.04 −0.15 −0.21 0.00

3.83 2.64 2.86 3.12 2.93

0.88 0.29 0.14 0.01 0.04

0.03 −0.02 −0.07 −0.13 0.00

2.71 1.54 1.80 2.04 1.93

1.04 0.41 0.14 0.02 0.03

0.02 0.03 −0.04 −0.03 0.00

1.63 0.45 0.80 0.94 0.92

1.16 0.51 0.15 0.01 0.03

0.00 0.48 0.00 0.00 0.00

0.59 0.49 0.00 0.00 0.00

1.26 0.87 0.00 0.00 0.00

Table 3. Relative Energies for Dissociation Limit and Reactant Complex of MO+-ZSM-5 with the Si → Al Substitution at T1, T2, and T12 Sitesa ΔE (kcal/mol) T1 site T1 site T2 site T2 site T12 site T12 site (HS) (LS) (HS) (LS) (HS) (LS) dissociation limit reactant complex

0.0 −4.5

dissociation limit reactant complex

0.0 −3.5


0.0 −1.8


0.0 −1.1

FeO+-ZSM-5 1.0 0.0 −1.9 −3.3 CoO+-ZSM-5 7.9 0.0 6.7 −3.0 NiO+-ZSM-5 4.6 0.0 3.9 −1.1 CuO+-ZSM-5 7.0 0.0 5.0 −1.9

2.7 1.6

0.0 −3.2

1.6 0.9

9.0 7.5

0.0 −3.2

8.5 6.9

8.3 6.1

0.0 −1.6

6.7 5.7

6.7 5.6

0.0 −1.5

7.4 6.1

a

See also Figure 1. HS and LS stand for high-spin state and low-spin state, respectively.

in the high-spin state. [CuO(CH4)]+-ZSM-5 with the Al atom at the T2 site in the high-spin state is found to be more stable by 0.8 and 0.4 kcal/mol than that with Al atom at the T1 and T12 sites in the high-spin state, respectively. Such slight differences in energetics of the reactant complex suggest that different T sites for the Si → Al substitution insignificantly affect the binding energy of methane. Mechanism of Methanol Formation by MO+-ZSM-5. In the second half of the reaction, as shown in Scheme 1, an abstraction of the CH3 moiety from the metal center and a 8326


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Figure 4. Optimized structures of TS2 and product complex for the methane to methanol conversion by (a) FeO+-ZSM-5, (b) CoO+-ZSM-5, (c) NiO+-ZSM-5, and (d) CuO+-ZSM-5 in the corresponding ground state. Some atoms of the zeolite’s unit cell are omitted for clarity. Bond lengths are given in Å. Color code: blue (Si), green (Al), red (O), gray (C), white (H), orange (Fe), purple (Co), yellow (Ni), cyan (Cu). A table showing the complete geometrical parameters in the high-spin and low-spin states is available in Supporting Information.

complexes of [Fe(CH3OH)]+-, [Co(CH3OH)]+-, [Ni(CH3OH)]+-, and [Cu(CH3OH)]+-ZSM-5 in the low-spin state are calculated to be 3.07 (1.46), 3.00 (1.46), 2.98 (1.46), and 2.93 (1.46) Å, respectively. The formed CH3OH molecule is coordinated to the metal center through its O atom with Fe−O, Co−O, Ni−O, and Cu−O bond lengths of 1.97, 1.93, 1.92, and 1.89 Å, respectively (Figure 4). As shown in Table 2, slight decreases in the O atom spin density from 0.14, 0.14, and 0.15, respectively, for the [HOFe-CH3]+-, [HOCo-CH3]+-, and [HONiCH3]+-ZSM-5 intermediates in the low-spin state to 0.04, 0.03, and 0.03, respectively, for the [Fe(CH3OH)]+-, [Co(CH3OH)]+-, and [Ni(CH3OH)]+-ZSM-5 product complexes in the low-spin state are evident from such HO−CH3 bond formation. Owing to the HO−CH3 recombination, the formed neutral methanol molecule left the metal atom with a positive charge to compensate for the negatively charged ZSM-5 zeolite. However, the Fe(I) and Co(I) are in general rather unstable oxidation states. Consequently, as shown in Figure 2, the energy levels of [Fe(CH3OH)]+-ZSM-5 and [Co(CH3OH)]+ZSM-5 product complexes in the low-spin state (−2.4 and −12.8 kcal/mol, respectively) are quite high, even higher than those of the corresponding hydroxo intermediate in the lowspin state (−23.0 and −23.7 kcal/mol, respectively). This also explains the high energy level for TS2 of FeO+-ZSM-5, in comparison with that for the corresponding TS1. To avoid such formation of the unfavorable Fe(I) and Co(I) species in the mononuclear model, we previously suggested that the decomposition of N2O can be added to the reaction intermediate when the concentration of N2O is sufficiently high.22 In the final complex M+-ZSM-5 + CH3OH, the adsorbed methanol is released from the Fe+-, Co+-, Ni+-, and Cu+-ZSM-5 zeolite surfaces with desorption energies of ΔEMeOH = 30.3, 35.3, 32.8, and 36.9 kcal/mol, respectively (Table 1), which are not significantly different from each other. This suggests that the variation of metals in ZSM-5 zeolite does not significantly influence the desorption energy of methanol. In Table 1, the ΔETS2 value for FeO+-ZSM-5 (37.3 kcal/mol) is found to be higher than the corresponding ΔEMeOH value (30.3 kcal/mol), indicating that the recombination of CH3 and OH moieties is the rate-determining step of the reaction.

recombination between the separating CH3 moiety and the metal-bound OH moiety occur via TS2 to form a product complex of methanol. As shown in Figure 4, the separating M···CH3 (and the approaching H3C···OH) distances for TS2s of FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 in the low-spin ground state are calculated to be 2.50 (1.89), 2.30 (1.89), 2.13 (1.86), and 2.08 (2.01) Å, respectively. Table 1 shows that the relative activation energies for such dissociation and recombination via TS2 (ΔETS2) are 37.3, 33.5, 19.0, and 10.2 kcal/mol, respectively. If we take the dispersion correction into account, those values become 38.6, 35.0, 20.5, and 12.9 kcal/mol, respectively, which are quite similar to the uncorrected values. To evaluate the selectivity of methanol, we compare the relative energy for methanol formation via TS2 with that for the formation of methyl radical, a competitive byproduct in the conversion of methane to methanol,77 via the reaction [HOM− CH3]+-ZSM-5 → M(OH)+-ZSM-5 + CH3•. As shown in Figure 2 (green lines), the relative energies for methyl radical formation by FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 are 14.9, 19.3, 12.9, and 1.1 kcal/mol, respectively, which are higher by 0.6, 9.5, 23.9, and 28.8 kcal/mol, respectively, than the relative energies of the corresponding TS2 in the low-spin state. This suggests that the formation of a methyl radical is highly disfavored for the reaction by CoO+-, NiO+-, and CuO+-ZSM-5, while the formations of methanol and methyl radical by FeO+-ZSM-5 are energetically competitive. On the basis of these results, we can thus predict that the selectivity of methanol from methane conversion by MO+-ZSM-5 would increase in the order FeO+ZSM-5 < CoO+-ZSM-5 < NiO+-ZSM-5 < CuO+-ZSM-5, which agrees very well with the experimental data showing that the selectivity of methanol from methane conversion by Fe-, Co-, and Cu-ZSM-5 is 12−77%,8,17 50−75%,14 and 83−98%,11,17 respectively, and correlates well with the methanol branching ratios of 41%, 100%, and 100%, respectively, for methane conversion by bare FeO+, CoO+, and NiO+ in the gas-phase reaction.38 In the product (methanol) complex, the CH3OH molecule is formed through a recombination between the separated methyl and the metal-bound OH moiety. The separated M···C distances (and the formed HO−CH3 bond lengths) for the product 8327


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are completely in contrast with ours, which show that methane is only weakly adsorbed on the MO+-ZSM-5 with a bent structure of O−M−CH4 (∠O−M−C ≪ 180°; see also Figure 3). As described above, the binding energy of methane varies inversely with ∠O−M−C. Thus, it is clear that the bent structure of O−M−CH4 formed in the reactant complex of MO+-ZSM-5 weakens the adsorption energy of methane. Such a bent structure can be easily understood by considering the MO+ species which is coordinated to two lattice oxygen atoms of the zeolite framework in such a way that it is surrounded by atoms of the 10-membered ring (see Figures S2−S5 in the Supporting Information, reactant complex). Such a structure thus confines the free space for methane to interact with the MO+ species and drives the methane molecule to form such a bent configuration. This result is consistent with previous work by Yumura et al.,27 showing that O2 intermediates generated during O2 activation by 2Cu-ZSM-5 are destabilized due to the zeolite confinement effect. Due to their high catalytic activities in a wide range of industrial applications,83 zeolites are expected to give metal cations additional catalytic functions. Thus, it is also interesting to compare our calculated C−H bond activation energies with those calculated for bare MO+ in the gas phase.41 As shown in Table 1, our calculated ΔETS1 values for TS1s of FeO+-, CoO+-, NiO+-, and CuO+-ZSM-5 in the corresponding ground state are 12.9, 17.3, 15.8, and 6.4 kcal/mol, respectively, which are expectedly lower by 9.2, 13.6, 9.6, and 26.4 kcal/mol, respectively, than those in the gas-phase process by bare FeO+, CoO+, NiO+, and CuO+.41 Yoshizawa et al. predicts FeO+-ZSM-5 in a cluster model to activate methane with a C−H bond activation energy of 16.0 kcal/mol,21 which is also lower than that by bare FeO+ in the gas-phase process.41 These results show that ZSM-5 zeolite indeed promotes the catalytic activity of MO+ cations toward methane. Conversion of Ethane to Ethanol by FeO+-ZSM-5. Similar to the methane to methanol conversion, the ethane to ethanol conversion also proceeds in a two-step manner via two transition states, where the activation of the reactant (ethane) takes place in the first half of the reaction via TS1 and the formation of the product (ethanol) takes place in the second half of the reaction via TS2. Figure 5 (reactant complex) shows that the ethane molecule is preferably adsorbed and activated in the sextet spin state with a calculated binding energy EB of −9.6 kcal/mol, which is reduced to −17.9 kcal/mol if the dispersion correction is taken into account and is lower by 5.1 kcal/mol than the calculated binding energy of methane in the sextet ground state, suggesting that ethane more strongly interacts with FeO+-ZSM-5. Figure 6 shows the calculated Fe···C1 and Fe···H distances of the [FeO(C2H6)]+-ZSM-5 reactant complex in the sextet ground state of 2.61 and 2.18 Å, respectively. After significant activation by FeO+-ZSM-5, the ethane molecule is decomposed into the C2H5 moiety and H atom. The dissociated H atom then approaches the O(oxo) atom via TS1 and at the same time the resultant C2H5 moiety migrates more closely to the Fe atom. In Figure 5, the spin inversion from the sextet state to the quartet state is observed before TS1, suggesting that the H−C2H5 bond dissociation by FeO+ZSM-5 occurs along with the spin inversion. This is similar to the H−CH3 bond dissociation by FeO+-ZSM-5 described above (Figure 2). The separating C···H distance and the approaching O···H distance of TS1 in the quartet ground state are calculated to be 1.33 and 1.37 Å, respectively (Figure 6).

However, in contrast, Panov et al. reported that the oxidation of methane at 200 °C results in products (methanol and dimethyl ether) accumulated on the Fe-ZSM-5 surface, while the product desorption into the gas phase occurs at above 200 °C,78 suggesting that the product desorption is the rate-determining step of the reaction. This disagreement, as described above, is due to the formation of the unstable Fe(I) oxidation state in the [Fe(CH3OH)]+-ZSM-5 product complex, which may underestimate the measured ΔEMeOH value. Therefore, for the sake of comparison, it is more reasonable to measure ΔEMeOH and ΔETS2 values for FeO+-ZSM-5 from the dissociation limit in the ground state. As shown in Figure 2, such relative energies are ΔEMeOH = 27.9 kcal/mol and ΔETS2 = 14.3 kcal/mol, respectively, showing that methanol desorption by Fe+-ZSM-5 is indeed the rate-determining step of the reaction, as suggested by Panov et al.78 The ground state of the final complex is not reinverted back to the high-spin state, causing the reactant and final complexes to have different spin multiplicities. This suggests that a spin inversion from the low-spin state to the high-spin state is expected to occur in the N2O decomposition by MO+-ZSM-5. The overall reaction of the methane to methanol conversion by NiO+- and CuO+-ZSM-5 is 1.6 and 23.0 kcal/mol exothermic, respectively, while that by FeO+- and CoO+-ZSM-5 is 27.9 and 22.5 kcal/mol endothermic, respectively. Confinement Effect of ZSM-5 Zeolite on the Reactivity. The framework type MFI of ZSM-5 zeolite with a medium 10-membered-ring size (free diameters of 5.4 × 5.6 Å and 5.1 × 5.4 Å in the [010] and [100] directions, respectively) is known to possess distinct steric effects and sieving properties for molecules having kinetic diameter >6.9 Å.64 Owing to a small kinetic diameter of about 3.7 Å,79 methane is easily adsorbed on and diffused into the ZSM-5 zeolite. However, when the methane molecule reaches the active site located on the wall of the 10-membered ring of ZSM-5 zeolite, it interacts with the MO+ species in a confined space due to the nanopores of ZSM-5 zeolite. Such an effect from zeolite nanopores is called a confinement effect by Zicovich-Wilson and Corma.80,81 The basic idea of this effect is that the orbitals of the molecule inside the zeolite cage are not extended over all the space, as they are in the gas phase, but instead within the limits of the zeolite cage.81 In this subsection, we investigate the effect of confined spaces on the adsorption and activation of methane over MO+-ZSM-5 by comparing our calculation results with those calculated for bare MO+ species in the gas-phase cluster model.40,41 It is noteworthy that MO+-ZSM-5 and bare MO+ in the gas-phase cluster experience different ligand fields. Moreover, the results for bare MO+ in the gas phase40,41 were obtained by using a different functional, i.e. B3LYP, which shows a rather different behavior for molecular interactions.82 Thus, a direct comparison between energetics of the gas-phase and zeolite models is rather difficult. Nevertheless, such a comparison, as suggested by Corma et al.,81 is important to provide useful information on the general trend of reactivity toward methane activation in the zeolite systems. In the gas-phase process, as shown in Table 1, methane is bound to the bare MO+ species with calculated binding energies EB of −22.8, −25.4, −28.9, and −45.8 kcal/mol,41 respectively, for H4C-FeO+, -CoO+, -NiO+, and -CuO+ complexes in the highspin ground state. Further, the methane molecule is adsorbed on the active site with such a configuration that the O, M, and C atoms are in line (∠O−M−C ≈ 180°). These calculation results 8328


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and O−H bonds with calculated bond lengths of 1.98 and 0.97 Å, respectively. Owing to such formations of covalent bonds, the energy level of the hydroxo intermediate in the quartet state, relatively from the dissociation limit of the ground state, is quite low (−28.1 kcal/mol), even lower by 5.1 kcal/mol than that for the methane to methanol conversion by FeO+-ZSM-5. In the second half of the reaction, the hydroxo intermediate is converted to product (ethanol) complex through the separation of the C2H5 moiety from the iron center and the recombination of the OH moiety with the separating C2H5 moiety via TS2. The separating Fe···C1 and the approaching C1···O distances of TS2 in the quartet ground state are calculated to be 2.57 and 1.95 Å, respectively (Figure 6). The activation energy for such bond dissociation and recombination via TS2 in the quartet ground state is calculated to be ΔETS2 = 32.7 kcal/mol, measured from the hydroxo intermediate of the ground state. This value is lower by 4.6 kcal/mol than that for the formation of methanol from methane by FeO+-ZSM-5 in the quartet state, suggesting that the conversion of hydroxo intermediate into product complex is more facile than the corresponding process in the methane to methanol conversion by FeO+-ZSM-5. The formation of the competitive byproduct, i.e. ethyl radical, proceeds in a reaction pathway of [HOFe-C2H5]+-ZSM-5 → Fe(OH)+-ZSM-5 + CH2CH3•, and the relative energy for this reaction is calculated to be 35.3 kcal/mol, measured from the hydroxo intermediate of the ground state (Figure 5, green line). This value is slightly higher by 2.6 kcal/mol than the obtained ΔETS2 (32.7 kcal/mol). Nevertheless, such a slight energy difference is still higher than the difference between energies for methanol and methyl radical formations (0.6 kcal/mol), suggesting that the product selectivity in the direct conversion of ethane to ethanol by FeO+-ZSM-5 is higher than that for the methane to methanol conversion by the same catalyst. In the product (ethanol) complex, the Fe···C1 distance is lengthened to 3.04 Å, indicating a complete separation of the C2H5 moiety from the Fe center, and the C1−O distance is shortened to 1.48 Å, indicating a full recombination of the OH and C2H5 moieties to form an ethanol that is bound to the iron center with an Fe−O bond length of 1.97 Å (Figure 6). This ethanol complex in the quartet state, as shown in Figure 5, lies −13.6 kcal/mol relatively from the dissociation limit in the ground state, which is more stable by 11.2 kcal/mol than the methanol complex formed by FeO+-ZSM-5 in the quartet ground state. Due to this low-lying energy level of product complex, the desorption energy for ethanol (34.3 kcal/mol) is higher than that for methanol (30.3 kcal/mol). Despite such slightly higher desorption energy, the overall reaction of the ethane to ethanol conversion by FeO+-ZSM-5 is less endothermic than that of the methane to methanol conversion by FeO+-ZSM-5 (20.7 versus 27.9 kcal/mol).

Figure 5. Potential energy diagram for the ethane to ethanol conversion by FeO+-ZSM-5 along the reaction pathway in the sextet (blue lines) and quartet (red lines) states. Values in parentheses are relative energies which include dispersion correction. The green line shows the relative energy for the formation of ethyl radical Fe(OH)+-ZSM-5 + CH2CH3•. All energies are given in kcal/mol.

The calculated activation energy for the H−C2H5 bond dissociation via TS1 in the quartet ground state, as shown in Figure 5, is ΔETS1 = 13.3 kcal/mol, which is quite similar to that for the H−CH3 bond dissociation by FeO+-ZSM-5 (12.9 kcal/mol). However, because ethane has a lower C−H bond strength than methane47 and the recent experimental results showed that Fe-ZSM-5 produces more oxygenates from ethane than from methane,17,48 the relative activation energies measured from the dissociation limit to TS1 in the corresponding ground state (3.7 and 8.4 kcal/mol, respectively for ethane and methane activations), for this case, provide more reasonable measures of C−H bond activation energies. As shown in Figure 6, the [HOFe-C2H5]+-ZSM-5 hydroxo intermediate is formed through the formations of covalent Fe−C1

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CONCLUSIONS The energetics and mechanisms of the direct conversion of methane to methanol by MO+-ZSM-5 zeolite (M = Fe, Co, Ni, Cu) as well as the role of ZSM-5 zeolite in that reaction have been studied by using DFT calculations on periodic systems. The van der Waals forces and three different locations for the Si → Al atom substitution of the zeolite have been considered in the investigations. On the basis of the calculated activation energies, we found that the reactivity toward C−H bond dissociation of methane is predicted to increase in the order CoO+-ZSM-5 < NiO+-ZSM-5 < FeO+-ZSM-5 < CuO+-ZSM-5,

Figure 6. Optimized structures of reactant complex, TS1, hydroxo intermediate, TS2, and product complex for the ethane to ethanol conversion by FeO+-ZSM-5 in the ground state. Some atoms of the zeolite’s unit cell are omitted for clarity. Bond lengths are given in Å. Color code: blue (Si), green (Al), red (O), gray (C), white (H), orange (Fe). A table showing the complete geometrical parameters in the high-spin and low-spin states is available in the Supporting Information. 8329


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while the selectivity of methanol is predicted to increase in the order FeO+-ZSM-5 < CoO+-ZSM-5 < NiO+-ZSM-5 < CuO+ZSM-5. However, in contrast, such high dependence on metals does not apply to the methanol desorption from the zeolite surface. The role of ZSM-5 zeolite in the catalytic activity of MO+ species has been investigated by comparing our calculation results with those reported for the gas-phase reaction by bare MO+ complexes.41 We found that the nanopores of ZSM-5 zeolite confines the free space for methane to interact with the MO+ species, which thus results in a significant destabilization of methane adsorption. As a result, the activation energy for the C−H bond dissociation of methane is lowered significantly. In addition to the direct conversion of methane to methanol by FeO+-ZSM-5, we have investigated the direct conversion of ethane to ethanol by the same catalyst for comparison. As expected, we found that the activation energy for H−C2H5 bond cleavage is lower than that for H−CH3 bond cleavage. Moreover, the product selectivity is also found to be higher for the conversion of ethane to ethanol with an overall reaction energy of 20.7 kcal/mol, which is less endothermic than the conversion of methane to methanol (27.9 kcal/mol).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01721. Relative energies of the initial MO+-ZSM-5 complexes with Al atoms at different T sites, optimized local structures of the initial MO+-ZSM-5 complexes with Al atoms at the T1 site, atomic charges and spin densities for selected atoms of the intermediates and transition states in the high-spin and low-spin states, and optimized local structures and geometrical parameters of reactant complexes, TS1, hydroxo intermediates, TS2, and product complexes in the high-spin and low-spin states (PDF)

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AUTHOR INFORMATION

Corresponding Author

*K.Y.: tel, 81-92-802-2529; fax, 81-92-802-2528; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI grant numbers JP24109014 and JP15K13710 from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the MEXT Projects of “World Premier International Research Center Initiative (WPI)”, “Integrated Research Consortium on Chemical Sciences”, “Network Joint Research Center for Materials and Devices”, “Elements Strategy Initiative to Form Core Research Center”, and JST-CREST “Innovative Catalysts”. M.H.M. gratefully acknowledges the Indonesia Endowment Fund for Education, the Ministry of Finance of Indonesia, for scholarship support.

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REFERENCES

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